The invention relates to a field bus arrangement with a distributor for field bus systems, which distributor can preferably be mounted in the area of a process engineering installation that is at risk of explosion.
Field buses are electrical apparatuses for communication between, on the one hand, field devices arranged in the area at risk of explosion, the so-called field area, examples of said field devices being sensors, actuators and measurement transducers, and, on the other hand, open-loop and closed-loop control systems arranged in the area not at risk of explosion, the so-called control room area. A field bus is usually designed as a two-wire line which simultaneously serves for transmitting the supply power for the connected field devices. In this case, the communication is effected in an analog fashion via a 0/4 . . . 20 mA current loop or digitally, for example by means of FSK modulation, or by combined analog/digital signal transmission.
For use in areas that are at risk of explosion, particular requirements are made of electrical operating equipment in order to preclude a possible explosion accident. In the context of installing and commissioning electrotechnical apparatuses and equipment and in the course of maintenance work on electrotechnical apparatuses and equipment in process engineering installations which, because of their intended use, are located in an atmosphere at risk of explosion, the observance of relevant statutory provisions such as the xe2x80x9cRegulation on electrical installations in areas at risk of explosionxe2x80x94ElexVxe2x80x9d, which is based on the European standards for explosion protection EN 50 014 ff., is binding.
On account of these statutory provisions, intrinsically safe electric circuits, exclusively, said circuits being subject to the rules of EN 50 020 as xe2x80x9cintrinsic safetyxe2x80x9d type of protection, are allowed to unconditionally disconnect and connect electrical lines in the course of operation.
Intrinsically safe electric circuits are subject to power limitation, however, and are thus unsuitable for the connection of devices and apparatuses with a power demand which exceeds the predetermined limit value.
In the case of all other types of protection, prior to manipulation on electric circuits, the entire installation must be disconnected from the voltage supply to an extent such that metallic components of the relevant electric circuits are isolated from voltage. In this case, the progress of the process proceeding on the process engineering installation is interrupted for the period of time between disconnection from the voltage supply and restart. The process then has to be started up anew. Operators of such process engineering installations find these interruptions extremely disruptive.
Safety barriers of multichannel design are known, e.g. from MTL (4700 series), in which each measurement channel is equipped with separate current limiting and separate voltage limiting. Safety barriers are regularly arranged in the area not at risk of explosion.
Furthermore, the patent specification DE 44 03 961 C2 describes a supply system for an intrinsically safe field bus, in which the means for current and voltage limiting are arranged such that they are spatially separate. What is disadvantageous in this case is the certification in accordance with the applicable statutory provisions on account of the inclusion of a plurality of devices in the certificate of conformity for explosion protection.
The invention is based on the object, therefore, of specifying a field bus arrangement with a field bus distributorxe2x80x94which can be mounted in the area of a process engineering installation that is at risk of explosionxe2x80x94for providing a multiplicity of connecting terminals of intrinsically safe electric circuits for power supply and data transmission for field devices having a bus capability, in which case the field bus distributor can be expanded to form an operationally ungrounded safety barrier.
According to the invention, this object is achieved by the means of Patent Claim 1. Advantageous refinements of the invention are specified in the claims referring back thereto.
The basic concept of this invention is the conversion and distribution of a large electrical power for supplying energy to field devices and also the reduction and limiting thereof to currents and voltages which enables the connection of certified explosion-protected operating equipment.
To that end, a field bus distributor which realizes the reduction of current and voltage to intrinsically safe values is arranged in the area of a process engineering installation that is at risk of explosion, the number of field devices that can be connected being as high as possible.
In detail, a high-energy voltage source is arranged in the area not at risk of explosion, the output power of which voltage source can be dimensioned independently of limitations prescribed by types of protection in accordance with the number of field devices to be connected and the line requirement thereof.
Furthermore, the main field bus run carrying exclusively communication signals appertaining to the data transmission is laid in the area not at risk of explosion.
The main field bus run and the voltage source are connected to a field bus coupler, which is preferably arranged in the area not at risk of explosion. The field bus coupler has three electric, circuits which are DC-isolated from one another, of which a first electric circuit is connected to the main field bus run. The field bus coupler is connected to the voltage source via a second electric circuit. The third electric circuit is routed as a non-intrinsically safe electric circuit into the area at risk of explosion and the supply voltage of the voltage source and the communication signals appertaining to the data transmission are applied to said third electric circuit.
The conductors of the non-intrinsically safe electric circuit are connected to terminals of a protected connection for connection of a non-intrinsically safe electric circuit of a field bus distributor arranged in the area at risk of explosion. This field bus distributor is equipped with n connections for intrinsically safe electric circuits for connection of a plurality of field devices. Each of the n connections for intrinsically safe electric circuits is connected to a busbar arrangement via a current limiting means. The protected connections for connection of a non-intrinsically safe electric circuit are connected to the busbar arrangement via voltage limiting means.
The field bus distributor has a housing-like encapsulation enclosing all the live components as a whole.
Each intrinsically safe electric circuit proceeding from the field bus distributor carries a voltage- and current-intensity-limited supply power for the connected field devices and superposed communication signals appertaining to the data transmission.
It is advantageous that the outlay on cabling between apparatuses in the area not at risk of explosion and apparatuses in the area at risk of explosion is very low. In this case, the subject matter of the present invention avoids both the voltage limiting for each measurement channel in the case of multichannel safety barriers and the inclusion of a plurality of devices in a certificate of conformity, since, in the context of the field bus arrangement which is the subject matter of the invention, only the field bus distributor is inherently the subject of official certification. Consequently, the technical outlay for attaining the certification that is prescribed for the use of electrical operating equipment in the area at risk of explosion is limited to the minimum that is technically necessary.
The voltage limiting means are constructed from zener diodes which are redundantly reverse-connected in series and are connected in parallel with the non-intrinsically safe electric circuit and upstream of which at least one fuse is connected on the supply side. By virtue of the grounding of the node of the interconnection of zener diodes, the voltage limiting means can be expanded to form an operationally ungrounded safety barrier in a simple and advantageous manner and with additional components being dispensed with.